The proposed research continues an interdisciplinary effort to assess the role of bacterial toxins in determining the course and severity of endophthalmitis. The goal of this effort is to identify which bacterial factors make major contributions to ocular damage during infection; and to characterize these factors and the resulting tissue damage in sufficient molecular detail to allow development and testing of experimental therapies which will specifically target these toxins and thus limit their effects. During the initial period of support, a cytolysin produced by Enterococcus faecalis was shown to contribute reproducibly to retinal destruction, whereas no similar effect was observed in infections with specific isogenic mutants, defective only in expression of this cytolysin. The cytolysin was characterized in molecular detail and two sites were identified for potential therapeutic action -- extracellular activation of the cytolysin and attachment of the bacterium to ocular structures. Experiments have been designed in the present study to determine whether non-toxic peptides can be used to block either of these two critical steps in the activity of the cytolysin and its localization near the retina. Experiments have also been proposed to characterize the factors synthesized by the soil organism Bacillus cereus that contribute to the unique destructiveness observed in posttraumatic endophthalmitis caused by this organism. Cloned toxin genes will be insertionally inactivated in vitro, and then reintroduced into B. cereus to derive a set of specific, isogenic mutants defective only in production of a single toxin. The virulence of each of these attenuated strains will be compared with the wild-type parental strain in an experimental endophthalmitis model. As has been accomplished for endophthalmitis caused by E. faecalis, the host/parasite interaction that occurs in B. cereus endophthalmitis, and the contribution of its various toxins will be characterized in sufficient detail to allow information based toxin-targeting therapeutic strategies to be developed.